Desulfurization of ferrous metals



Dec. 11,- 1951 P. M. HULME DESULF'URIZATION OF' FERROUS METALS 2 SHEETS-SHEET 1 Filed Nov. 4, 1949 h s F at v.

INVENTOR PHILIP M. HULME ATTORNEYS Dec. 11, 1951 HULME 2,577,764

DESULFURIZATION 0F FERRQUS METALS Filed NOV. 4, 1949 2 SHEETSS HEET 2 INVENTOR PHILIP M. HULME ATTORNEYS Patented Dec. 11, 1951 UNITED STATES PATENT omcs 2,577,764 DESULFURIZATION OF FEBBOUS METALS Philip M. Hulme, summa, Conn., aesig'nor to Air Reduction Company, Incorporated, New York, N. Y., a corporation of New York Application November 4, 1949, Serial No. 125,607

g 2 Claims. a I

This invention relates to an improved method for rapidly and eiiiciently reducing the sulphur content of ferrous metals.

Sulphur is an undesirable element which is always present in ferrous metals. The sulphur is derived from the ore, the scrap or' the fluxes making up the charge and from the fuel used, and it may also enter the metal from other sources. In recent years, the demand for iron and steel of relatively low sulphur. content has increased markedly. At the same time, due to exhaustion of better materials, the available ore, scrap, fluxes and fuels have been of such quality that molten iron and steel produced from them have tended to be considerably higher in sulphur content than formerly. Thus, the necessity for a practical and economical method for reducing the sulphur content of the metal is presented.

It has been known for many years that calcium carbide is an excellent desulphurlzing agent for iron and steel, since it combines readily with the sulphur present therein. Nevertheless, it has not been utilized because of the difficulty arising from the fact that calcium carbide does not melt at the temperatures of molten iron and steel. The reaction must be effected between a solid reagent and the liquid molten metal. It depends, therefore, upon surface contact between the solid calcium carbide and the moltenmetal. The problem of securing suflicient contact to effect the desired reaction rapidly and economically has been heretofore insurmountable. 5 T

It has been proposed to blow the calcium carbide, in a finely divided conditioned and suspended in a stream of gas, into the molten metal. Such a suspension requires the use of a large amount of gas in proportion to the carbide supplied. The suspended carbide is principally carried in the bubbles of gas which rise through the molten metal and never comes into contact therewith. Most of the carbide used is thus ineffective, and the low efliciency of the procedure makes it impractical. Other previous proposals for introducing carbide to molten metal are equally ineffective or impractical because they fail to assure the maximum surface contact between the reagent and the metal or for other reasons.

Thus, it has been realized that calcium carbide is a good desulphurizer, but it has not been possible to develop a suitable method permitting the carbide particles to come into intimate contact with the molten metal. On page 317 of the 1946 Handbook of Cupola Operation (American 2 Foundrymens Association) the following statement appears:

Calcium carbide (CaCz) is an active desulphurizing agent, although its use in foundries is only of academic interest. To ensure uniformly eillcient results, the reagent must be introduced below the surface of the bath or thoroughly stirred into the iron by mechanical means, both of which are impractical in large scale commercial operations.

It is the object of the present invention to provide an eflective and economical method for disseminating calcium carbide in molten ferrous metal so that the sulphur reacts almost completely with the carbide to effect its removal.

Other objects and advantages of the invention will be apparent as it is better understood by reference to the following specification and the accompanying drawing, in which:

Fig. 1 is an elevation partially in section showing an apparatus suitable for the practice of the present invention in a batch operation; and

Fig. 2 is a similar view illustrating the application of the invention in a continuous operation involving the use of a melting furnace or cupola and a forehearth where the desulphurization of the molten metal is effected.

Actual experimentshave revealed the fact that the proportional amount of gas used heretofore in the effort to introduce calcium carbide to molten metal is such that the gas in which the carbide is suspended merely serves to carry most of the carbide through and out of the molten metal before it can accomplish its purpose. The major part of the carbide never becomes sufliciently wetted by the molten metal to effect the intimate contact necessary for its reaction with the sulphur. In accordance with the present invention, therefore, while a gas is employed, it is not used as a carrier for the carbide, Its purpose is merely to prevent the molten metal from rising in the submerged passage through which the carbide is delivered beneath the surface of the molten metal. The carbide is fed regularly and in predetermined amounts and at a predetermined rates and falls by gravity through the passage and is thus disseminated throu h the molten metal as it rises therethrough. While the gas-will escape and rise as bubbles through the molten metal, the gas does not carry any substantial portion of the carbide in suspension because of the relatively low ratio of gas to solid and thus does not remove it from effective contact with the metal before the reaction canbe accomplished.

The experiments have shownthat the of not more than'one-quarter totwo cubic feet of as per pound of calcium carbide is ample for the purpose. rovided the gas is maintained at a pressure sufficient to overcome the static head, of the molten metal and thus prevent entrance thereof into the passage throu h which the calcium carbide descends b gravity. It has been found that when the calcium carbide and the gas are empoyed in the proportions indicated.

it is possible to effect a better than 90% extraction of the sulphur from the molten metal with 35% eiliciency of-the carbide reaction. This is in marked contrast with the procedure as herethe metal. It is obvious that the 'use of two cubic feet of gas per pound of carbide greatly reduces the chi ling eflect in comparison with the conventional method, which requires approx mately 100 cubic feet of gas per pound of carbide.

Another limiting condition required to effect the desired result is that the, apparent linear velocities of the gas passing through the conduit shall not exceed 25 linear feet per second. If it exceeds that limit, an exces ive amount of the solid material is carried rapidly to the surface and consequently it does not have an opportunity to be wetted by the molten metal and to r act therewith. The apparent linear velocity is the cubic feet of gas passing through a conduit per second divided by the cross-section of the conduit, regardless of temperature, pressure and compressibility factors of the gas involved and irrespective of the quantities of solids passing through the same conduit.

The operationcan be carried out readily in any suitable receptacle. This is illustrated in Fig. 1, in which a receptacle 5 carries a charge 5 of molten metal for treatment in a batch operation. To facilitate the treatment. 'a frame I is mounted on the plunger 8 of a hydraulic cylinder 9 which is suitably supported on a base II. The frame I carries a tube II in which a worm I2 is adapted to be driven by a shaft l3 connected by a coupling 4 to the shaft E5 of a motor It, the shaft l5 being driven through a suitable variable speed mechanism ll.

A hopper I8 is adapted to contain a charge of carbide IS in suitable finely divided form. The carbide descends from the hopper into the tube H and is advanced by the worm l2 to the end of the tube H, where it falls by gravity into-a -tube 20 which extends into a heat-insulated and refractory member 2|. The latter. when the apparatus is lowered to the position indicated in Fig. 1, extends well into the molten metal and the carbide is disseminated therethrough.

In order to prevent the molten metal from rising in the member 2|, a suitable gas is provided in a container such as a cylinder 22 which is connected through a flowmeter 25 to a pressure-reducing valve 23 having a gauge 24 which indicates the pressure. The gas is delivered as required through a pipe 23 which extends to a point adjacent the end of the worm [2. Thus, gas is supplied at the proper pressure to the tube Il and the member 2| through which the carbide is fed to the molten metal. A by-pass pipe 21 with a valve 21' connects the pipe 28 to the closed cover 28 of the hopper I! so that the gas pressliige Ii: equalized in the chamber above the carb e As hereinbefore indicated, the pressure of the gas supplied'ismaintained at a point which will overcome the static head of the molten metal in the member 2|, thus preventing the molten metal from rising in the passage through the member 2|, leaving the carbide free to fall by gravity to the lower end of the member 2|. The volume of gas supplied is restricted so as to maintain the desired pressure and the proper ratio of gas to the amount ofcarbide fed. Thus, the carbide enters the molten metal and, because it is lighter, tends to rise therethrough, but no substantial proportion is carried away in the gas bubbles as in th procedure heretofore suggested and employed. Consequently the sulphur has ample opportunity to react with the carbide in the molten metal and the desulphurization is effected rapidly.

Referring to Fig. 2, the invention may be applied to the continuous desulphurization of molten ferrous metal which is supplied from a melting furnace or cupola 29 which is adapted to deliver the metal continuously at a predetermined rate through the opening 30 provided for that purpose. The details of the furnace formno part of the present invention and are well understood in theart.

The molten metal is delivered into a slag skimmer 3|, having a baflle 32 to eliminate the slag, and thence into the forehearth 33 having a baffle 34 with an opening 35 at the bottom thereof and a baille 31, the latter having an opening 38 at the bottom thereof. The baflle 31 prevents the slag and the reaction product of the carbide from passing to the outlet spout 39 of the forehearth. The slag may be withdrawn as required through an opening 40 in the wall of the forehearth.

To accomplish the purpose of the present invention, a frame 4| is supported on the plunger ceding embodiment of the invention, which is driven by a shaft 45 connected to a variable speed motor 45. Calcium carbide is maintained in a hopper 41 mounted on the support 41, and the carbide is delivered through a flexible tube 48 to the tube 44. It is delivered by the worm to a pipe 43 which is connected to the heat insulated and refractory member 50 which extends through the roof of the forehearth 33 and may be lowered so that its lower end is submerged in the molten metal.

A cylinder 5| affording a supply of gas is connected through a pressure-reducing valve 52 to a pipe 53 which extends through a control valve 54 and pressure indicator 55 to the tube 44, thus supplying gas at suitable pressure to prevent the molten metal from rising in the member 50. A by-pass pipe 55 delivers gas to the hopper 41 to maintain a balanced pressure on the carbide in the hopper 41.

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not act as a carrier for the carbide but merely prevents the molten metal from rising in the passage through which the carbide descends into the molten metal.

Since the sulphur reaction with carbide in the molten metal is essentially a solid to liquid reaction, the particle size of the carbide is important. The smaller the particle size, the greater the surface area is in proportion to the weight of the carbide. This fact, plus the feature of having the particles completely wetted by the molten metal, produces a high reaction efliciency. I have found that the particles must be finer than mesh and that preferably at least 50% of the total carbide should be finer than 48 mesh. Actually, the only limitation on the extreme fineness of the carbide particles is their practicable flowability. Since the particles of carbide are light compared to the liquid metal, they rise rapidly towards the surface. While the reaction rate between carbide and the sulphur is relatively rapid, it does require some time for the reacting substances to come in contact. Therefore, the longer the wetted carbide can be kept in direct contact with the metal, the higher the efliciency. In order to gain sufllcient time of contact, the particles should travel a substantial distance through the metal. Hence the depth of immersion of the conduit has 8. definite and direct bearing on the efliciency of the reaction. For example, if the conduit through which the carbide passes is immersed only one inch below the surface, the reaction efficiency is in the order of 1 to 2%. When the conduit is immersed at least ten inches, the efficiency is increased to approximately 35%. In any event, the end of the conduit should extend at least six inches into the molten metal and preferably more than one half of the depth of the molten-metal. Thus, in the operation, it is preferable to use a furnace, ladle or other equipment which is relatively deep. This facilitates longer contact time of the carbide with the metal.

The following data shows the results obtained:

With the proposed carbide practice, the total treatment time consists of approximately ten minutes, with actual injection time of one minute--the remaining time is necessary to permit the dispersed particles of reaction products to rise to the surface. The more finely divided the particles and the more thoroughly dispersed, the more time is necessary for them to separate from the molten metal. In the continuous operation as described in connection with Fig. 2, the rate of flow of the molten metal can be regulated readily to allow ample time for the reaction and separation. In using the forehearth, the size and shape of the vessel and the internal construction must be such that an ef-. ficient process is realized. Each amount of metal passed per minute must be retained in the forehearth sufliciently long to allow the reaction and separation to occur. Assuming that the total treatment time shall consist of ten minutes, and a 10-ton per hour operation is being conducted, the rate of flow per minute will be or 333 pounds per minute. Therefore the container in which desulphurization takes place should be capable of holding 3330 pounds of molten metal in order to allow for the inverse settling of the removed sulphur which comes to the surface of the metal in combination with the calcium carbide introduced.

In accordance with the invention, exceptionally low levels of sulphur content in molten ferrous metal may be attained in a comparatively short time. The advantages are obvious in a great many cases. For instance, in open hearth operations, depending on cupola iron for hot metal, the cupola iron runs anywhere from .17 to .20 per cent sulphur, and when a final specification of .025 is required, at least two hours of extra refining is needed in the open hearth. Also, several times the normal lime and ferromanganese additions must be made to meet the specification. Now, if the cupola iron is originally reduced to less than .010 in sulphur, no time is required in the open hearth for desulphurizing, less lime and ferromanganese are required, and the total slag volume is one-third or one-half that in the former practice. and as a result, the metal loss is considerably lower because of the smaller quantity of iron oxide in the slag. Also, if the cupola iron has a low sulphur content of approximately .005 sulphur, the mere mixing of it with molten scrap of higher sulphur content results in a mixture well below specification.

For the purpose of the invention, various gases may be used, provided the selected gas is dry and oxygen-free, such as a gas inert with respect to the molten metal or any non-oxidizing gas. The most readily available and cheapest gases are nitrogen, carbon dioxide, propane and natural gas. The precise composition of the gas is immaterial so long as it does not enter into any chemical or oxidizing reaction with the molten metal. Nitrogen, for example, could be employed, although the cheaper gases are just as eificient.

While the method is primarily applicable to the treatment of molten ferrous metal, it may be utilized to reduce the sulphur content of other metals.

An important advantage of the invention is that there is no reversal of the reaction of the sulphur with the carbide. Once the sulphur has combined with the carbide, it is carried from the metal and the sulphur does not return to the metal as in the case of other reagents.

Various changes may be made in the procedure and apparatus as described without departing from the invention or sacrificing the advantages thereof.

I claim:

1. The method of treating molten ferrous metal containing sulphur with calcium carbide to produce a reaction between said sulphur and v said calcium carbide which comprises feeding through said duct and into said molten metal.

separately feeding finely divided particles of calcium carbide into said gas stream for substantially continuous introduction therewith into the molten metal. and regulating the feeding of said calcium carbide independently of the feeding of said gas but in predetermined relationship thereto to maintain the flow of calcium carbide relative to the flow of gas in a ratio within the range of from about one-fourth to about two cubic feet of gas per pound of calcium carbide, whereby the particles of said calcium carbide are wetted by said molten ferrous metal as said particles rise therethrough.

2. The method of treating molten ferrous metal as defined in claim 1 in which the calcium carbide is introduced into a body of molten metal, molten metal is continuously introduced into said body, the reaction products of the calcium carbide with sulphur of the metal and unreacted calcium carbide are permitted to rise continuously to the surface of said body, and molten metal is withdrawn from said body at a point 8 therein which is substantially free of said prodnets of reaction and unreacted calcium carbide.

PHILIP M. HULME.

REFERENCES CITED The following references are of record in the i'ile of this patent:

UNITED s'rA'rEs PATENTS OTHER REFERENCES Desulphurization of Pig Iron with Calcium Carbide. Published as TP1131 in Metals Technology, December 1939, and in Transactions, A. I. M. E., 1940, vol. 140, pages 87 to 105, inc. 

1. THE METHOD OF TREATING MOLTEN FERROUS METAL CONTAINING SULPHUR WITH CALCIUM CARBIDE TO PRODUCE A REACTION BETWEEN SAID SULPHUR AND SAID CALCIUM CARBIDE WHICH COMPRISES FEEDING AND INERT GAS AT A REGULATED RATE OF FLOW INTO A DUCT HAVING AN OUTLET AT A SUBSTANTIAL DISTANCE BELOW THE SURFACE OF SAID MOLTEN METAL TO PROVIDE A SUBSTANTIALLY CONTINUOUS FLOW OF SAID PROTHROUGH SAID SUCT AND INTO SAID MOLTEN METAL, SEPARATELY FEEDING FINELY DIVIDED PARTICLES OF CALCIUM CARBIDE INTO SAID GAS STREAM FOR SUBSTANTIALLY CONTINUOUS INTRODUCTION THEREWITH INTO THE MOLTEN METAL, AND REGULATING THE FEEDING OF CALCIUM CARBIDE INDEPENDENTLY OF THE FEEDING OF SAID GAS BUT IN PREDETERMINED RELATIONSHIP THERETO TO MAINTAIN THE FLOW OF CALCIUM CARBIDE RELATIVE TO THE FLOW OF GAS IN A RATIO WITHIN THE RANGE OF FROM ABOUT ONE-FOURTH TO ABOUT TWO CUBIC FEET OF GAS PER POUND OF CALCIUM CARBIDE ARE WETTED THE PARTICLES OF SAID CALCIUM CARBIDE ARE WETTED BY SAID MOLTEN FERROUS METAL AS SAID PARICLES RISE THERETHROUGH. 